Gas separation: processes – Filtering – And reduction of pressure
Reexamination Certificate
1999-03-08
2001-06-26
Smith, Duane S. (Department: 1724)
Gas separation: processes
Filtering
And reduction of pressure
C055S341100, C055S417000, C055S418000, C055S431000, C055S432000, C055S467000, C096S379000, C096S399000
Reexamination Certificate
active
06251169
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a vacuum/reduced pressure refining process and a vacuum/reduced pressure refining facility for use in metal refining, i.e. refining of alloys such as steel, etc., e.g. molten metal, etc. in a vacuum/reduced pressure converter, a degassing apparatus for a vacuum ladle, etc.
2. Description of Related Art
In continuous operation of vacuum/reduced pressure refining treatment under a subatmospheric pressure, the following steps are repeatedly carried out in series to exchange the treated molten metal in the refining vessel with an untreated molten metal.
That is, in case of a vacuum/reduced pressure refining apparatus of such a model as shown in
FIG. 8
, at first molten metal
13
is charged into a refining vessel
1
, followed by closing the vessel with a lid
14
and reducing the inside pressure of the refining vessel to vacuum/reduced pressure. In case of a vacuum/reduced pressure refining apparatus, as shown in
FIG. 9
, a ladle
17
containing the molten metal is placed in a refining vessel
1
, followed by closing the vessel with a lid
14
and reducing the inside pressure of the refining vessel to vacuum/reduced pressure. In the case of a vacuum/reduced pressure refining apparatus, as shown in
FIG. 10
, a ladle
17
containing molten metal
13
is positioned on a ladle elevating device
18
under a refining vessel
1
, followed by dipping the lower end
19
of the refining vessel
1
into the molten metal and reducing the inside pressure of the refining vessel to a vacuum/reduced pressure. After the vacuum/reduced pressure treatment, the inside pressure of the vacuum/reduced pressure refining vessel is returned to atmospheric pressure, followed by removing the lid from the refining vessel to open the vessel or by removing the lower end of the refining vessel from the molten metal in the ladle. Then, the treated molten metal is discharged from the refining vessel, or the ladle is taken out. The period from the end of these operations to the start of the next treatment is a waiting period.
Use of a filter-type dust collector in a vacuum evacuating apparatus is known, for example, from JP-A-617115. In such a system the dust collector must be connected to a vacuum/reduced pressure refining vessel and used in a hermetically closed state during the vacuum/reduced pressure refining treatment, and thus there is no suction of excess air therein during the treatment. When dust in an unoxidized metallic state is generated in the vacuum/reduced pressure refining vessel, the dust reaches the dust collector, while maintaining the unoxidized state. As a result, when air invades the dust collector for reasons such as pressure returning to the atmospheric pressure by air, etc., the metal dust deposited on the filter react with the air, thereby causing the problems of oxidation/heat generation. As a result, in the case of filter cloth being used for a filter, the filter cloth is damaged by heat or completely burnt in serious cases. In the case of ceramics being used for a filter, the filter itself undergoes no direct damage by heat, but the collected dust is sintered to cause clogging in the filter meshes or impair the filterability of the filter due to solidification on the filter.
To solve these problems, JP-A-8-3627 discloses that, in the case that combustible substances are contained in the dust, the dust collector must be subjected to pressure returning or back washing with an argon gas or a nitrogen gas to prevent the filter from being damaged by the air introduced at the time of pressure returning after the vacuum degassing treatment of treated molten metal.
Problems to be Solved by the Invention
1) The problem of filter damage at the time of pressure returning to the atmospheric pressure just after the vacuum/reduced pressure treatment can be solved by the above-mentioned measures, but any such measures have not been so far applied until the start of the next treatment, etc. That is, even if back washing with an argon gas, a nitrogen gas or the like is carried out after the treatment, all of the dust captured on the filter cannot be separated and cannot fall down, and a portion of the dust may still remain attached to the filter until the start of the next treatment. In the case where the remaining dust contains unoxidized fine powders of metal of high oxygen affinity such as magnesium, etc., we have another problem of filter damage at the start of the next treatment, even if the pressure returning with an argon gas, a nitrogen gas or the like is carried out.
Specifically, filter damage as a result of suction of a large amount of air from the open connection port at an upstream side (refining vessel) of the dust collector into the dust collector at the start of vacuum/reduced pressure refining treatment, for example, from the open port of an expansion joint unconnected to the refining vessel, the open port of the refining vessel out of lid engagement, the lower ends of RH dipping pipes, etc. occurs, for example, when reduced pressure evacuating apparatus
4
is to be started before expansion joint
9
is connected, as in the case of a vacuum/reduced pressure refining facility having expansion joint
9
between vacuum lid
14
and upstream duct
5
, as shown in
FIG. 4
, or when reduced pressure evacuating apparatus
4
is to be started before vacuum lid
14
is completely engaged in a vacuum,/reduced pressure refining facility as shown in
FIG. 9
, or when reduced pressure evacuating apparatus
4
is to be started before suction pipes
19
are dipped into molten metal
13
by elevating ladle
17
by elevating device
18
in the vacuum/reduced pressure refining facility, as shown in FIG.
10
.
Furthermore, when the refined molten metal is to be exchanged with untreated molten metal during the operation of vacuum/reduced pressure treatment, the process of returning the pressure to atmospheric pressure is carried out, and then the lid is released from the refining vessel or the bottom end of the vessel is released from the molten metal to exchange molten metals. During the exchangement or the waiting period between one treatment and another, atmospheric air invades the refining vessel and duct
5
connecting the refining vessel to the dust collector is open to the atmosphere. A simplified duct is shown in
FIG. 4
, etc., but actually a gas cooler, a cyclone separator, etc. (not shown in the drawings) are provided in the duct. That is, the actual duct often has a large net capacity. Thus, at an initial stage of the treatment, not only the air introduced from the outside by suction, but also the air remaining in the duct extending from the dust collector to the refining vessel is led to the dust collector to oxidize the remaining dust on the filter to generate heat and damage the filter in some cases.
2) Thus far, no measures have been taken to prevent filter cloth from being damaged due to oxidation of dust deposited on the filter cloth or to prevent ceramic filter from clogging, or to o prevent the apparatus from damage and dust discharge trouble due to oxidation or sintering of dust accumulated in the lower part of the dust collector through separation and falling down, caused by suction of atmospheric air from the dust discharge port during the vacuum/reduced pressure treatment. That is, vacuum sealing is carried out by providing a valve, a lid or the like for vacuum sealing at the dust discharge port, but its sealability is likely to deteriorate due to the dust, and leakage is more likely to occur than at other positions of the vacuum/reduced pressure refining facility. When the degree of leakage is considerably high, the filter is damaged by the oxygen in the introduced air by suction during the vacuum/reduced pressure treatment. Even if the degree of leakage is not high enough to directly damage the filter, the dust accumulated at the lower part of the dust collector through separation and falling down is oxidized by the oxygen to generate heat and causes problems such as vacuum seal damage and discha
Abe Kenji
Iwasaki Hiroshi
Makino Nobuyuki
Morishige Hiroaki
Ogawa Gaku
Nippon Steel Corporation
Smith Duane S.
Wenderoth , Lind & Ponack, L.L.P.
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